A user of stereo headphones often perceives simple monaural or a pure stereo source of music played through headphones as emanating from a point somewhere between the two headphones which, when the headphones are being worn, is somewhere inside their own head. Even if the location of the source is not perceived as being from within one's own head, the sound is often perceived as unnatural and fixed at a location that is not like what is felt to be normal compared to what is expected when the same music is listened to with a speaker system and also unlike what is desired by the sound engineer or artist who recorded the music. Undesirable effects such as these are more pronounced using headphones than with larger speaker systems because sound emanating from a speaker system is typically reflected and dampened by multiple walls or other objects which surround the listener, and this bouncing and mixing of signals provides for a presence that is typically more pleasing and natural sounding than that heard when a simple stereo amplifier is used to drive stereo headphones. The perception of the location of the source and the pleasantness of that sound when listening with headphones is a complex issue that is a result of the signal processing effects and/or techniques which have been found to be pleasing to a typical listener. These pleasing effects have often been measured and selected by empirical means, i.e., selecting them based upon some predetermined standard.
In general, the desired effect of the signal processing is to move the perceived source of the sound away from the apparent point halfway between the headphones, to places which are some distance from the listener and which may be located either in front of or behind the listener. Often, different frequencies or instruments with different sounds are perceived to be emanating from different locations.
In a recording studio, when music is recorded, there are typically more than two tracks or signals recorded when a performance is being played by musicians for recording. A typical number of tracks may be 32 or 16. It is sometimes the case that all the tracks are not recorded at the same time. A trumpet player may be recorded on one track, the bass drum on another, a vocalist or backup singer may be on other tracks and in similar fashion tracks are assigned for other instruments or sounds. The decision as to which sound or sounds are placed on each track is up to the recording engineers. In order to produce a recording for distribution to the public, these tracks are then mixed together to form a recording with a smaller number of tracks, typically two for stereo, in which the levels of the different tracks are selected by the recording engineer or producer and then mixed and phased onto the smaller number of tracks to achieve the desired sound. The same track may appear at different levels on more than one channel of the production output channels. This process is usually called the mix-down process. Since the number of tracks that existed originally is larger than the result after the mix-down there is typically a loss of information, that is, not all of the musical signals present in all 32 tracks (for example) will be present in the final, typically stereo, channels. The levels and sounds chosen for inclusion, and the phasing of those signals on the production output is largely an art as these levels are selected by practical experience and artistic feeling.
When played back in a stereo environment, the two channels are typically described as LEFT and RIGHT, or L and R channels, respectively. These two channels are typically played through the left and right speakers of a stereo speaker system, or through the left and right headphones of a headphone system. When played through speakers, the source of the sound that will be perceived by a typical listener can be predictably moved by the recording engineer to any point between the two speakers during the mix-down by making the level of sound from a chosen track louder on one speaker than the other. If the left or “L” channel is louder the signal will appear to move towards the left or “L” speaker. If the levels are equal between “L” and “R” the sound will typically be perceived as being located near the center point between the speakers. When listening with headphones, however, moving the perceived source of sound is more difficult, and typically requires circuitry which adjusts the phase or level of signals in an attempt to achieve the same or similar effects to that which are achieved more easily or naturally with actual speakers in a room with walls, a ceiling and a floor. The translation and/or prediction of what is individually perceived by the listener is not always completely predictable.
It is therefore a broad object of this invention to improve the perceived sound of a personal stereo audio system utilizing a circuit for amplification which includes feedback that is based upon the difference between the LEFT and RIGHT channels of the stereo signal. It is a second broad object of the invention to implement the audio enhancement circuit in a manner which is simpler than that used in prior art implementations of circuits with similar capabilities. It is a third broad object of the invention to implement a circuit that is more reliable than that of the prior art. It is a fourth broad object of the invention to provide for the sharing with another listener of the enhanced stereo sound from a personal listening device incorporating the circuit.
The circuit is intended primarily for use in providing a better perceived quality of sound when listening to a stereo audio signal with headphones and also to provide for the sharing of that enhanced stereo audio signal with another headphone listener. The circuit can also be used as part of small stereo sound systems such as might be found attached to a personal computer or other personal listening devices. The circuit utilizes the difference signal formed as the difference in magnitude of the LEFT and RIGHT stereo signal. This difference signal, when fed at varying levels and phasing into the mix of sound for each ear in a headphone, has been found to result in a movement in the perceived source of the sound and provides for an overall enhancement of the listening experience. The exemplary circuit of the invention uses fixed values for forming the cross-feed signal which has been determined empirically to be good for a majority of listeners while maintaining the simplicity of the circuitry. The circuitry of the invention is also simpler than that of the prior art resulting in the potential for increased reliability and lower production cost.
The invention also provides capability for the sharing of the audio signal with another person using headphones by providing multiple output headphone jacks.